I think the article says iron levels are negatively correlated to longevity, but "the association might be due to a already-known correlation to haemochromatosis" which I have no idea what that is. I presume it's some known population that's old that happens to also have an enrichment of haemochromatosis but where individuals in that population without it do not show a positive defect in longevity?
Edit: specified as negatively correlated